This invention relates generally to electrical resistance bodies used as fluid heaters having a plurality of passages extending therethrough for reception of fluid to be heated by the bodies and more particularly to such bodies which are operable with multiphase electrical power.
Resistors having passages extending through a body of ceramic resistance material of positive temperature coefficient of resistivity have been proposed for use as fluid heaters. Such heaters are safe and self-regulating and are adapted to generate high volume outputs of heated air for a hair dryer or the like or to safely and efficiently heat the air-fuel mixture being supplied to an automobile engine to assure early volatilization of the fuel before the mixture is furnished to the engine. In one such device, coatings are formed on the inner walls of the resistor body passages and are interconnected so that electrical current can be directed through the thin webs of resistance material between coatings of opposite polarity in adjacent body passages, thereby to generate a large amount of heat for efficient transfer to a fluid directed through the passages. A device of this type is shown and described in U.S. Pat. No. 4,264,888.
Another use for this type of heater is shown and described in copending application Ser. No. 852,484, now U.S. Pat. No. 4,678,982, assigned to the assignee of the instant invention and filed on even date herewith. In that application a heater particularly adapted for use in an automotive vehicle is energized by a three phase power supply derived from the alternator of the vehicle. One of the uses of the heater is as a supplemental air heater wherein the heater is disposed in the air stream generated by the main heater fan downstream of the main heater. The supplemental heater is used, when desired, to add incremental heat to the passenger compartment to greatly reduce warm up time for creating an environment comfortable for passengers, or in vehicles having marginal heat sources for adding sufficient heat to the passenger compartment to achieve a selected steady state temperature on particularly cold days. The heater is comprises of a plurality of discrete heater bodies made in accordance with U.S. Pat. No. 4,264,888 mounted in a frame which is adapted to be placed across an air duct intercepting air carried from the main heater. Each of the bodies is of a ceramic material or the like of positive temperature coefficient of resistivity (PTC) having a large member of passages extending through the body in side-by-side parallel relationship to each other between opposite ends of the body. The passage walls define the webs of resistance material between adjacent passages in the body and the walls are of uniform thickness from end to end extending to the generally flat ends of the resistor body. Electrically conductive metal coatings are adhered to the inner walls of the body passages of the resistor body. The passages are divided into two groups with the passages of one group alternated among the passages of the other group and electrically conducting interconnecting means are provided to interconnect the passages of each group together.
The bodies are arranged in the frame and electrical interconnecting means are provided so that each phase of the current is directed to its own body or bodies with current directed through the thin webs of resistance material located between adjacent body passages in each respective body.
The particular resistance of the web is a function of heat dissipation of the webs which in turn is a function of the air flow passing through the passages. Due to a number of factors, air flow through a duct tends to be somewhat uneven across its cross section, however, to achieve an efficient, long lived system it is important to have an even resistive load among the phases to avoid adversely affecting the alternator. The heater assembly disclosed in the copending application obtains a balanced load by arranging the bodies into a plurality of rows of bodies connected to each phase and alternating the rows so that the bodies of each phase will be in several locations across the cross section of the duct. The bodies within each phase are electrically connected in parallel relation.
While the above described arrangement can be effective, the ability to achieve a satisfactory resistive balance requires a certain amount of custom work in providing proper row alternations for air streams of different ducts. Further, if the air flow profile changes, for example with different fan speeds, then some imbalance may result. In addition, it is desirable to reduce the number of component parts of the assembly in order to minimize manufacturing costs.
It is therefor an object of the present invention to provide a multipassage, multiphase, self-regulating fluid heater which is simple yet reliable and which results in a balanced resistive load for power generation regardless of variations in the flow of the fluid stream which pass through the passages.